I would like to get some confirmation that if we replace A53/A72 with A7/A9 while the SoC architecture is still based on ARM v8 bus & IPs (e.g. CCI400, NIC400, TZC400, GIC500, BP141), does such design feasible and workable?
The ARM Architecture (e.g. ARMv7 ARMv8) isn't bound to a particular version of the bus architecture. The bus protocols are managed by a separate specification - the AMBA Specifications. There are a significant number of different AMBA Specifications, which describe bus protocols which have different use cases.
A processor implementing a certain version of the ARM Architecture might use one of a number of different AMBA bus interfaces, and may support multiple different ones.
The IPs that you describe will implement particular AMBA interfaces that may or may not be compatible with a certain processor. The various features that the IP supports will also have an impact as to whether a design is feasible. For example, CCI-400 support hardware coherency. If you intended to build a big.LITTLE system, then you would likely need a hardware coherent interconnect. CCI-400 is compatible with some ARMv7 and ARMv8 implementations.